With the demand increasing for more complex functions and higher performance in an integrated circuit, there is need to pack as many active devices as densly as possible. Density depends upon not only the area required to isolate one transistor from another and but also the active area of the transistor itself. One conventional method of isolation is the well-known local oxidation of silicon (LOCOS) technology in which active regions are separated by a locally grown field oxide. According to this technology, the degree of isolation depends on the length and depth of the field oxide layer separating the active regions. By increasing either the length or the depth of the field oxide layer, the length of the leakage path between the two active areas is also decreased. However, to increase circuit density, the length of the field oxide layer must also be minimized. Increasing the depth of the field oxide induces a side effect where the edges of the field oxides encroach into the neighboring active regions. The encroachment results in a deformation phenomenon known as "bird's beak".
In order to prevent the "bird's beak" phenomenon, various methods have been provided.
FIGS. 1A and 1B are partial cross-sectional views showing the formation process of a field oxide for the isolation of devices. Referring to FIG. 1A, there is prepared a wafer 10 having a thermal oxide 2, polysilicon layer 3, and silicon nitride layer 4 thereon in that order, wherein the substrate 1 contains impurities therein. The layers stacked on the substrate 1 is prepared as follows.
The substrate 1 is loaded into a diffusion furnace having a high temperature such as 850-950.degree. C. Next, O.sub.2 gas and H.sub.2 gas are introduced into the diffusion furnace to form a thermal oxide film 2 with a thickness of approximately 150-250 Angstroms on the silicon substrate 1, and then the substrate 1 is withdrawn from the diffusion furnace.
Subsequently, the substrate 1 with the thermal oxide film 2 thereon is loaded into a first low pressure chemical vapor deposition (LPCVD) chamber wherein SiH.sub.4 gas is introduced therein. Thereafter using the thermal decomposition of the SiH.sub.4 gas, a polysilicon layer 3 having a thickness 400-600 Angstroms is formed on the thermal oxide film 2, wherein the polysilicon layer 3 buffers the stress between silicon substrate 1 and silicon nitride layer 4 being formed. Afterwards, the substrate 1 is withdrawn from the first LPCVD chamber.
Subsequently, the substrate having the thermal oxide 2 and polysilicon layer 3 thereon is loaded into a second LPCVD chamber in order to deposit a silicon nitride film 4. A NH.sub.3 gas and dichloro-silane (DCS:SiH.sub.2 Cl.sub.2) gas are introduced into the second LPCVD chamber, to form a silicon nitride film 4 with a thickness of approximately 1,000-2,000 Angstroms on the polysilicon layer 3.
Afterwards, using the photolithography process, a photoresist pattern (did not shown) is formed on the silicon nitride film 4 so that a predetermind portion of the field oxide layer is exposed. Thereafter the silicon nitride film 4 and the polysilicon layer 3 are etched by using the photoresist pattern, and the photoresist pattern is removed by conventional method.
Finally, the exposed portion is oxidized using the patterned silicon nitride film 4 as a mask to form a field oxide layer 5 as shown in FIG. 1B.
The above-mentioned isolation method, however, has a problem in that the distribution of doped impurities in a silicon substrate 1 is varied with the formation of the thermal oxide. This is because the silicon atoms in the silicon substrate having an impurity distribution of a steady state diffuse out from the surface of the silicon substrate during the formation of the thermal oxide and react with oxygen atoms. Accordingly the distribution characteristic of the doped impurities in the silicon substrate varies.
In addition, the method has a shortcoming in that the surface of the substrate is polluted during the loading and unloading steps between the chambers for the formation of the thermal oxide, polysilicon layer, and silicon nitride layer since each layer is formed at a different deposition apparatus.